1. Field of the Invention
The present invention relates to a double-flow gas friction pump including a housing having suction and discharge ports, a shaft supported in the housing and extending at a right angle to an axis of the suction port, a rotor fixedly secured on the shaft for joint rotation therewith, and a stator fixedly secured in the housing and cooperating with the rotor for pumping gas.
2. Description of the Prior Art
In a conventional gas friction pump of a Gaede type a cylindrical rotor rotates in a cylindrical housing having an annular groove which is broken in one location. For increasing the pressure ratios, several stages are arranged one after another. In a modified embodiment of this gas friction pump, in the Holwerk gas friction pump, instead of several stages a spiral groove is provided. In another type of a gas friction pump, in the Siegbahn pump, the spiral grooves are provided on opposite sides of a disc-shaped rotor. All of the above-mentioned pumps are characterized by high pressure ratios. Therefore, these pumps in particular, the Siegbahn pump, are particularly suitable for applications in which a high fore-vacuum pressure exists. However, because of a narrow stator-rotor split, their suction capacity is limited. A higher suction capacity is provided by turbomolecular pumps, the turbine-shape construction of which provides for a larger discharge volume.
Molecular and turbomolecular pumps may have a single-flow or a double-flow construction. The advantage of a single-flow pumps consists in that the connection flange and, thus, the receiver of the discharge gas are directly attached on the high vacuum side of the pump rotor. Therefore, the pumped gas can be taken over directly by the pumping elements, without a substantial flow resistance, and conducted further.
The drawback of the double-flow pump consists in that the gas stream from the suction flange should be deviated in order to reach the pumping elements of the pump. This is associated with a high flow resistance and, thus, with large losses of suction capacities. However, the double-flow pumps have a basic advantage with respect to the single-flow pumps which consists in that conventional ball and magnetic bearings of different constructions permit to easily achieve the stability criteria required in the double-flow pumps. At that, the bearings and the drives are always located on a fore-vacuum side of the double-flow pumps, so that no impairment of the high vacuum with these elements takes place.
The suction region of a double-flow pump, in addition to bearing means and a drive, includes a double pumping surface for delivery of a gas. However, their advantage can only be partially used because, as it was discussed above, of a deviation of the gas stream, high flow losses take place.
Accordingly, an object of the present invention is to provide a double-flow gas friction pump in which the double pumping surface available in the suction region of the pump can be used more advantageously.